A. Field of the Invention
This invention relates to machines and methods for automatically bending or shaping patient-specific orthodontic archwires, retainers, or other orthodontic or medical devices to a configuration having a desired geometry.
B. Description of Related Art
In orthodontics, a patient suffering from a malocclusion is treated by affixing brackets to the surface of the teeth and installing an archwire in the slots of the brackets. The archwire and brackets are designed to generate a customized force system that applies forces to teeth, by which individual teeth are moved relative to surrounding anatomical structures into a desired occlusion. The most common approach to creating such a force system is to use off-the-shelf brackets, with or without built-in standardized prescription values, and designing a customized archwire that has complex bends designed to move or rotate the teeth in the desired direction.
These complex bends are characterized by superposition of rotation along the length axis, change in angles in two planes orthogonal to the length axis, and displacements in the length axis and transverse to the wire. In general, these deformations are described by six degrees of freedom. In orthodontics, the change in angles are specified as torque, rotation and angulation. The displacements are specified according their direction relative to the teeth geometry as mesial-distal, buccal-lingual and occlusal-gingival.
The wire segment to be deformed is smaller than the distance between the nearest edges of the bracket slots of the brackets bonded to two adjacent teeth and is usually in the range of only a few millimeters.
Machines for bending orthodontic archwires have been proposed in the prior art. Andreiko et al., in U.S. Pat. No. 5,447,432, describes an apparatus that takes a straight archwire and imparts a simple planar arcuate curvature to the wire. The wire is customized in the sense that the shape of the arc is designed for a particular patient, but the wire bending apparatus described in Andreiko et al. is limited to a customized bracket approach to orthodontics. In particular, the Andreiko et al. wire bending apparatus cannot produce any complex bends in the wire, e.g., bends requiring a combination of translation and rotational motion. The required force system is created by virtue of the design of customized brackets, and in particular slot positions and orientations in the brackets and their exact placement on the teeth in desired positions, such that the brackets interact with the flat planar wire to move teeth into desired positions.
The patent to Orthuber et al., U.S. Pat. No. 4,656,860, describes a bending robot for bending archwires. A robot as described in the '860 patent was developed by the predecessor of the assignee of the present invention and used experimentally for several years, but never widely commercialized. The robot consisted of two characteristic design features: a bending cone that could move forwards and backwards to bend the wire, and a rotating cone that could twist the wire. Accordingly, this machine could perform, within a single bending step, only a deformation around a main axis of the wire. To do a complex deformation of the archwires with six degrees of freedom as commonly required for orthodontic patient-specific archwires, the machine of the '860 patent would typically require a sequence of five single bending steps, comprising three torque (twisting) steps and two bending steps. The archwire segment length necessary to accomplish these plurality of steps usually exceeds the available distance between the nearest edges of the two adjoining bracket slots. Hence, the '860 patent would not work for many orthodontic prescriptions. Additionally, the '860 bending machine was rather imprecise because the robot has no effective feedback mechanism for detecting how the wire in fact was bent after a particular bending or twisting operation was performed. Furthermore, manufacturing inaccuracies of each bending step lead to a relative large error in the whole deformation between two adjacent bracket slots.
The deficiencies in the '860 patent led the present assignee to develop an improved wire bending machine, described in the patents of Werner Butscher, et al., see U.S. Pat. Nos. 6,612,143; 6,732,558; 6,755,064; 6,860,132; 7,076,980 and 7,283,891. The entire content of these patents is incorporated by reference herein. These patents describe an articulated robot arm with six consecutively arranged rotation axes capable of movement in six degrees of freedom to bend an orthodontic wire. To perform the wire deformation, the wire is gripped by both a fixed gripping tool mounted to a table or base, and by a gripping tool mounted at the end of a robot arm. By appropriately moving the robot arm, the segment of the wire gripped between the two grippers is bent and/or twisted to a desired deformation. A variety of other configurations for the robot arm were proposed, including one based on a combination of translation and rotational axes (see FIG. 9 of the '143 patent).
The assignee's work related to wire bending machines as described in the above-referenced U.S. Pat. Nos. 6,612,143; 6,732,558; 6,755,064; 6,860,132; 7,076,980 and 7,283,891 has resulted in the inventors' appreciation that the robot described in these patents, while representing a substantial advance in the art, nevertheless has several shortcomings.
Firstly, the geometry of the bends that can be manufactured is limited and does not allow for creation of all the wire shapes desired by orthodontists. In cases where the bend length must be very short, such as in lingual cases, the treatment options can be limited by this approach.
Secondly, highly accurate archwires are difficult to obtain in a repeatable manner by the robot described in these patents. To guarantee the desired accuracy of finished archwires, the archwires must be measured after the bending has been completed. If the deviations between actual and desired shapes exceed tolerance limits, the wire must be re-bent (i.e., a new wire created). Bending correction values are calculated from errors measured during the first manufacturing run, and are used during the second iteration to obtain a more accurate archwire. A third iteration could also be required in order to fulfill the tolerance limits at all bends of a certain archwire. This process wastes wires, increases production time, and increases manufacturing costs.
The present invention provides a machine for bending medical devices such as orthodontic archwires, and a method of bending such devices, that overcomes these shortcomings, thereby providing a further substantial advance of the prior art. The machines and methods of this invention enable the bending of orthodontic archwires with significantly improved precision with much wider range of possible shapes, and while lowering manufacturing costs.